1. Field of Invention
The present invention relates to a device for cooling by natural convection, with or without phase change. Specifically to such devices incorporating unique surface contours that provide a thermosyphon effect with jet-impingement cooling of the heat source.
2. Description of Prior Art
In many fields, the control and dissipation of excess thermal energy is desired. The speed and reliability of modern electronic equipment depend on the cost-efficient dissipation and control of waste heat. As component performance increases and size decreases, the heat flux (power/area) spirals upward.
Air has been used as a coolant since the origin of electronic devices. Gravity-dependant, natural air convection produces a heat transfer coefficient of about 5 to 30 W/m.sup.2 .degree. C. Using only natural convection with air, a typical 1 Watt, 45 millimeters per side, horizontally-mounted device, has a temperature rise (.DELTA.T) of roughly 45.degree. C. Forced air convection (3 meters per second velocity) can reduce the device's .DELTA.T to roughly 15.degree. C. As electronic components have required better heat dissipation techniques, designers have increased the power of air cooling systems. High velocity air systems reach a point of diminishing returns. Higher power cooling systems also generate their own high waste heat loads and high noise levels from air moving devices. These high heat loads and high noise levels must then also be controlled. The cost constraints in this approach are nowhere more apparent than in the field of personal computers. These systems use high performance VLSI (Very Large Scale Integration), CMOS (Complementary Metal-Oxide Semiconductor), and RISC (Reduced Instruction Set Computer) technologies. Some high heat flux electronic devices present a thermal obstacle that forced convection with air cannot overcome. Therefore, there is a great need to reduce an electrical device's temperature in the most efficient manner possible. CMOS devices especially, show great increases in performance and reliability when operated at reduced temperatures. Tobey teaches the use of reduced temperatures to gain increased reliability and speed in CMOS devices in U.S. Pat. 4,812,733 (1989).
Various methods enhance the heat transfer capabilities of forced air convection systems. Such methods are well known in the art and include enhanced surfaces, swirl flow, jet-impingement, vibration, injection or suction, and electrostatic or magnetic fields. These techniques are beneficial, but are limited by the physical properties of the ambient air, and can require significant mechanical apparatus.
Forced air convection heat transfer coefficients range from approximately 30 to 300 W/m.sup.2 .degree.C. An arrangement for air-impingement cooling of electronic modules is disclosed by Dunn et al. in U.S. Pat. No. 4,277,816 (1981). Dunn utilizes air, directed under pressure to the electronic modules. This device lacks a leak-proof air exhaust passage, therefore, the cross-flow degradation of the cooler inlet air with heated exhaust air causes a great loss of efficiency. Modules at the system air exhaust are exposed to exhaust air that has been heated by the previous modules. Hwang et al., in U.S. Pat. No. 4,233,644 (1980) teaches the use of a pair of air-moving devices to pull air through a plenum chamber. The air is then directed to impinge against the electronic modules. In Hwang's device, the air is directed to separate inlet and exhaust passages. In most instances, the use of this device requires a major level of redesign of the entire electronic cabinet, or the intention to use such a cooling scheme while designing the equipment. Hamadah et al., in U.S. Pat. No. 5,063,476, discloses an apparatus for controlled air-impingement cooling. This device utilizes controlled inlet and exhaust air channels contained in circuit board-sized modules. Hamadah's apparatus is thought to provide a lower level of redesign for an overheating electronic cabinet, but still suffers from the physical properties of air and the limitations of air-moving devices. U.S. Pat. No. 4,682,651, by Gabuzda (1987), describes a heat sink array for use with air-impingement systems. The novelty of this device is that the heat sink is segmented to minimize the effect of thermal contraction and expansion on the circuit packages. Thermal mismatch is a concern between the circuit board and the leaded side of a circuit package, as this mismatch stresses the solder joints. Gabuzda's device is only beneficial when the circuit package is so large, and the heat transfer so inefficient, that controlling the thermal mismatch on the non-leaded side of a component will have some effect on reliability.
Peltier junctions are often suitable for cooling discrete electronic devices below ambient temperature. The use of Peltier junctions in this manner is taught in U.S. Pat. No. 4,238,759 by Hunsperger, 4,685,081 by Richman (1987), and 4,812,733 by Tobey (1989). The obstacle to incorporating a Peltier junction into a design is the resultant condensate and the inefficiency of these device. By example to cool a 10-Watt device, an additional 25 Watts of heat may be developed by the Peltier junction itself, then requiring removal of 35 Watts of heat. Some computer designers have used a Peltier junction to cool the central processing unit (CPU). and then additional air-moving fans to cool the Peltier junction.
In 1935 experimenters reported using de-ionized water to cool components. Natural convection in water can produce heat transfer coefficients in the range from 300 to 2,000 W/M.sup.2 .degree.C. For the previously mentioned one Watt electronic device, the .DELTA.T is only 1.degree.C. when immersed in water. Special synthetic dielectric coolants have recently been developed for electronic cooling. Some of these are silicate ester, perfluorocarbon, and polyalphaolephin based compounds.
Many systems, such as the Cray-2 Supercomputer, have been designed to operate immersed in a perfluorocarbon fluid using natural convection. This technique is complicated by the properties of the fluid high volatility and low surface tension. These problems have been addressed in the industry by securing the perfluorocarbon fluid in flexible plastic bags. These bags are attached to the electronic circuit boards. This approach decreases the effectiveness of free convection by restricting the internal fluid motion, and also adds unwanted thermal impedance as the heat must pass through the thermally insulative plastic bag containing the fluid.
Forced liquid convection heat transfer coefficients are much higher than forced air (30 to 300 W/m.sup.2 .degree.C.), and range from approximately 300 to 6,000 W/m.sup.2 .degree.C. The prior art for liquid jet-impingement devices does not have the limitations of the properties of air, but still suffer from the drawbacks associated with liquid moving devices. In my own U.S. patent application, Ser. No. 674,820 of 1991, a flexible module is described that allows the controlled application of multiple jets of an impinging liquid coolant. This device is unique because novel features are used to seal the liquid from the electronic components while still delivering a degree of geometric flexibility, and the maximum amount of heat transfer. Bland et al., in U.S. Pat. No. 4,494,171 (1985), and Crowe, in U.S. Pat. No. 4,901,201 both describe very complex forced impingement cooling apparatus referred to by the acronym CHIC, for Compact High Intensity Cooler. Crowe's device offers a means to provide dedicated cooling for high heat flux devices located in the midst of other components requiring minimal cooling. Users of Crowe's invention must know the exact location of each of the high heat flux devices in order to use the CHIC in only that location, because a lower cooling level is provided to all other components. U.S. Pat. No. 4,750,086, by Mittal (1988), teaches the use of a forced liquid jet-impingement device that incorporates a bellow seal and a thick heat spreader. In all of the aforementioned jet-impingement devices, liquid and air, a pressurized media is used for forced-convection heat transfer. Forced convection requires an external power source to propel the coolant, in addition to the coolant moving device itself. These devices create waste heat and must be located to distribute the cooling media in an evenly distributed manner. This entails the use of plumbed lines and leak-proof modules. No device known, has used the ability of a natural convection thermosyphon to power the jet-impingement apparatus.
Although liquid cooling systems are much more efficient than air, they have been cost prohibitive in small systems such as personal computers, because of the necessary fluid-moving device, plumbed lines, and leak-proofing.
Cooling a heat source by means of phase change (boiling and condensing) is the most efficient method used in the industry. While single-phase forced liquid convection heat transfer coefficients range from 300 to 6,000 W/m.sup.2 .degree.C., boiling water heat transfer coefficients range from approximately 3,000 to 60,000 W/m.sup.2 .degree.C. and condensing steam systems operate in the range of 6,000 to 120,000 W/m.sup.2 .degree.C. Mitsuoka, in U.S. Pat. No. 3,986,550 (1976) describes a cooling device operating on the phase change principle that maintains separation between the liquid and gas phases of the coolant media. No attempt is made in Mitsuoka's device to increase the heat transfer coefficient by use of internal extended surfaces, jet-impingement techniques, or other geometric forms. In fact, in the described patent, Mitsuoka teaches away from impingement by using a variety of devices placed in the flowstream to prevent intermixing of the fluid and vapor phases of the coolant.
Within the prior art of natural convection, air-cooled heat sinks, it is not evident that a specific geometric form will channel the natural connection flowfield currents to gain greater heat dissipation through minimized thermal boundary layers, or that this geometric form (with adjustment for coolant properties) can be used in conjunction with natural convection liquid-immersed heat sinks. Although there are many examples of prior art single-phase thermosyphon devices, and prior art two-phase thermosyphon-powered cooling devices, it is clearly unobvious to those skilled in the art that there are advantages in heat transfer coefficient and (operating efficiency when the jet-impingement technique is combined with either or both of these two approaches, ie, single-phase thermosyphon powered jet-impingement, or two-phase thermosyphon-powered jet impingement. Further, in some thermosyphon-powered prior art (Mitsuoka), the return of the condensate to the boiling pool is shown as occurring in the geometric center of the heat source. The prior art does not recognize that specific novel geometric features that connect fluid transfer channels affect the heat transfer coefficient, and that significant departure from these unobvious geometric ratios will result in a device that may operate in a reverse mode, resulting in a severe degradation in performance. A review of the literature yields no discussion of these combinations, or the supposition that such combinations are advantageous or even possible.